As mammals evolved from reptiles some important changes occured in the jaws. In reptiles the lower jaw is made up of a number of bones, one of which, the dentary, carries the teeth. Growth of the lower jaw seems to be "sutural." Correlated to the mode of growth, the teeth of most reptiles are ankylosed and thus unable to adapt to a growth pattern which includes appositional growth on the ridges. When during evolution to mammals the growth center of the lower jaw is shifted to the newly evolved mandibular condyle, ankylosis of the teeth was replaced by syndesmosis with the development of the periodontal ligament. There are insuperable difficulties to ascertain the changes around the teeth in fossils. The Crocodilia offer a most fortunate object of investigation. Though not ancestral to Mammalia, they represent a stage in which the reptilian mandible is still preserved, but where the ankylosis of the teeth is replaced by snydesmosis through a periodontal ligament. It is evident that the reptiles on their way to mammals must have passed through a stage now represented by the Crocodilia because such a preadaptational stage was indispensible for the changes in the reptilian jaw. Since it is relatively simple to obtain living material, e.g. young caimans, several problems pertaining to evolution can be investigated. Specifically, 1. The muscles, jaws, and jaw joints integrating structure and function, 2. the detailed histology of the periodontium, 3. a clarification of the growth pattern of the reptilian jaw, and 4. an electron microscope study of the peculiar keratinized epithelial attachments seen in caimans, can be thoroughly investigated.